Diethylnitrosamine (DENA) was administered with drinking water (40 mg/liter) to male Wistar rats for 4, 6, 8, and 10 weeks. The protein:DNA ratio and the ultraviolet light spectral properties of liver chromatin were not modified by DENA treatment. Nonhistone proteins were separated by sodium dodecyl sulfate:polyacrylamide electrophoresis on slab gels and analyzed by densitometry with a scanning microphotometer connected on line to a computer. There were no qualitative changes in the pattern of nonhistone proteins during the treatment with DENA. The quantitative changes statistically significant at p less than 0.005 were detected only in the 4th and 10th week, increases in fractions with molecular weights of 41,000 to 47,000 and 51,000 to 64,000 and decreases in fractions with molecular weights of 27,000 to 32,000 and 47,000 to 51,000 having been found. The proteinase activity of liver chromatin was assayed in incubation mixtures with 0.2 and 2 M NaCl and the measurement of the cleavage products was performed with ninhydrin. Proteolytic activity was found only in 0.2 M NaCl and was higher in rats treated for 8 weeks with DENA than in controls. Autolysis of chromatin for 24 hr at 37 degrees showed a severe breakdown of the nonhistone protein, being greater in the high-molecular-weight fractions than in the low-molecular-weight fractions.